Assessment of spawning stock for walleye pollock <i>Gadus chalcogrammus</i> Pallas, 1814 in the northern Okhotsk Sea by ichthyoplankton method taking into account the eggs mortality

Abstract

A new approach for assessment of the spawning stock of walleye pollock in the northern Okhotsk Sea on the data of ichthyoplankton surveys is proposed. To estimate the actual number of spawned eggs, average age of eggs at each stage of development was determined and the eggs mortality was estimated in dependence on age. For the spring ichthyoplankton surveys in 2019–2023, the eggs mortality was evaluated as 27 % for stage I, 57 % for the stage II, 74 % for the stage III, and 86 % for the stage IV – these portions of the spawned eggs were not caught and counted. So, the number of counted eggs was in 1.7–2.1 times lower that the actual number of spawned eggs. The spawning stock biomass was amounted as 12.9 . 106 t in 2019–2023, on average, on the base of the actual number of eggs determined taking unto account the eggs mortality (while the biomass calculated from the counted number of eggs only would be 6.9 . 106 t). Results of the spawning stock estimation with this new method are comparable with the stock assessments by trawl surveys. The eggs number correction through mortality is recommended for calculation of the spawning stock for walleye pollock in the Okhotsk Sea using the ichthyoplankton method.